Propylene oxide (PO) is a versatile chemical intermediate used in a wide range of industrial and commercial products including polyether polyols, propylene glycols and propylene glycol ethers. By volume, it is among the top 50 chemicals produced in the world with the annual production of about 5 million tons. Industrial production of propylene oxide is mainly from co-oxidation of propylene with other chemicals but these technologies create additional side products. The major conventional manufacturing methods of PO are the chlorohydrins•process and the Halcon process. The chlohydrin process is being phased out because of environmental pollution, while the latter has the by product limitation. So a new technology with environmentally benign has to be developed for the production of propylene oxide. The main concern in the fine chemical and drug intermediates are the amount of waste generated per unit weight of desired product (called E-factor by R A Sheldon in Chemsitry & Industry, 6 Jan. 1997, P 13) and poor atom efficiencies (kg of product produced per Kg of reactants used) due to the use of stoichiometric reagents and minerals acid/base catalysts. In this context, the use of solid catalysts which are eco-safe and reusuable become important. Moreover a major problem with this process is that it produces phenol is driving its price down and also hurting the economics of phenol as well. This concern is the impetus for researchers to develop a direct single step co-product free and environment friendly route to propylene oxide.
There are reports on the production of propylene oxide by direct oxidation of propylene with different oxidants over different solid catalyst but to the best of our knowledge there is no reference for the use of molecular oxygen only for this purpose.
Reference may be made to article in the Science 2001, 292, 1139-1141 by Chinese group Zuwei et al the use of H2O2 as the oxidizing agent for the conversion of propylene oxide from propylene to achieve ˜85% yield over W containing heteropolyacid.
Reference may be made to article in the Journal of Catalysis 2002, 211, 552-555 by Can Li and his group reported the use of mixture of H2 and O2 for the oxidation of propylene to propylene oxide to achieve ˜1% conversion and 43% propylene oxide selectivity over Nacl-modified VCe1-xCux oxide catalyst.
Reference may be made to article in the Journal of Phys. Chem. B., 2005, 109, 19309-19319 by Nijhuis et al reported the use of H2+O2 as oxidants for the conversion of propylene to propylene oxide with ˜4% propylene conversion and 95% propylene oxide selectivity over Au supported titania catalyst.
Reference may be made to article in the Journal of Catalysis, 2005, 232, 85-95, in which Oyama et al and his group reported the use of molecular oxygen for the oxidation of propylene to propylene oxide to achieve ˜30 propylene conversion with ˜10% propylene oxide selectivity over Ag supported CaCO3 catalyst.
Reference may be made to article in the Angew. Chem. Int. Ed. 2004, 43, 1546-1548, in which Japanese worker Prof Haruta and his group reported the use of mixture of H2+O2 for the oxidation of propylene to propylene oxide to achieve 10% conversion and ˜90% selectivity over Ba(NO3)2-Au/titanosilicate catalyst.
Reference may also be made to article in the Angew. Chem. Int. Ed. 2006, 45 412-415, in which the same Japanese worker Prof Haruta and his group reported the use of mixture of H2+O2 for the oxidation of propylene to propylene oxide to achieve 9% conversion and ˜90% selectivity over gold supported titanosilicate catalyst, where small amount of trimethyl amine was introduced with the feed mixture.
Reference may be made to article in the Catalysis Lett. 2007, 119, 185-190 in which Lu et al reported the use of molecular oxygen as the oxidizing agent for the oxidation of propylene to propylene oxide to achieve 4% propylene conversion and 46.8% propylene oxide selectivity over Ag—Y2O3—K2O/Al2O3 catalyst.
Reference may also be made to article in the Journal of Phys. Chem. C, 2008, 112, 7731-7734 in which Wang et al reported the use of molecular oxygen as the oxidizing agent to convert propylene to propylene oxide with a propylene conversion of ˜4% with a propylene oxide selectivity of 55% over CuOx—SiO2 catalyst.
Reference may also be made to article in the Ind. Eng. Chem. Res. 2010, 49, 2614-2637 in which Bettina et al reported the use of nitrous oxide as the oxidizing agent for the oxidation of propylene to propylene oxide to achieve ˜10% conversion and ˜75% propylene oxide selectivity over Fe/SiO2 catalyst.
Reference may also be made to article in the Angew. Chem. Int. Ed. 2009, 48 1546-1548, in which the same Japanese worker Prof Haruta and his group reported the use of mixture of H2O+O2 for the oxidation of propylene to propylene oxide to achieve 0.8% conversion and ˜57% selectivity over gold supported TS-1 catalyst.
The drawback of the processes reported so far is that they do not exhibit sufficiently high conversions of propylene for high selectivity of propylene oxide to be of interest for industrial application. In most of the cases hazardous oxidizing agent N2O, H2O2 or expensive H2 with O2 was used and also lots of unnecessary by-products was formed. In addition, the catalysts used have a limited activity under the operating conditions. There is, therefore, an evident necessity for further improvements in the process for the selective conversion of propylene to propylene oxide.